Current materials research aimed at fabricating high-temperature superconducting ceramics in conductor configurations for bulk, practical applications, is largely focused on powder-in-tube methods. Such methods have proved quite successful for the Bi--(Pb)--Sr--Ca--Cu--O (BSCCO) family of superconductors due to their unique mica-like mechanical deformation characteristics. In high magnetic fields, this family of superconductors is generally limited to applications below 30.degree. K. In the Re--Ba--Cu--0 (ReBCO, Re denotes a rare earth element), Tl--(Pb,Bi)--Sr--(Ba)--Ca--Cu--O and Hg--(Pb)--Sr--(Ba)--Ca--Cu--O families of superconductors, some of the compounds have much higher intrinsic limits and can be used at higher temperatures.
It has been demonstrated that these superconductors possess high critical current densities (Jc) at high temperatures when fabricated as single crystals or in essentially single-crystal form as epitaxial films on single crystal substrates such as SrTiO.sub.3 and LaAlO.sub.3. These superconductors have so far proven intractable to conventional ceramics and materials processing techniques to form long lengths of conductor with J.sub.c comparable to epitaxial films. This is primarily because of the "weak-link" effect.
It has been demonstrated that in ReBCO, biaxial texture is necessary to obtain high transport critical current densities. High J.sub.c 's have been reported in polycrystalline ReBCO in thin films deposited on special substrates on which a biaxially textured non-superconducting oxide buffer layer is first deposited using ion-beam assisted deposition (IBAD) techniques. IBAD is a slow, expensive process, and difficult to scale up for production of lengths adequate for many applications.
High J.sub.c 's have also been reported in polycrystalline ReBCO melt-processed bulk material which contains primarily small angle grain boundaries. Melt processing is also considered too slow for production of practical lengths.
Thin-film materials having perovskite-like structures are important in superconductivity, ferroelectrics, and electro-optics. Many applications using these materials require, or would be significantly improved by, single crystal, c-axis oriented perovskite-like films grown on single-crystal or highly aligned metal or metal-coated substrates.
For instance, Y--Ba.sub.2 --Cu.sub.3 --O.sub.x (YBCO) is an important superconducting material for the development of superconducting current leads, transmission lines, motor and magnetic windings, and other electrical conductor applications. When cooled below their transition temperature, superconducting materials have essentially no electrical resistance and carry electrical current without heating up. One technique for fabricating a superconducting wire or tape is to deposit a YBCO film on a metallic substrate. Superconducting YBCO has been deposited on polycrystaliine metals in which the YBCO is c-axis oriented, but not aligned in-plane. To carry high electrical currents and remain superconducting, however, the YBCO films must be biaxially textured, preferably c-axis oriented, with effectively no large-angle grain boundaries, since such grain boundaries are detrimental to the current-carrying capability of the material. YBCO films deposited on polycrystalline metal substrates do not generally meet this criterion.
It has been demonstrated that high critical current densities can be obtained in YBCO films that have been epitaxially deposited on biaxially textured Ni. However, for practical superconductivity applications it would be desirable to have a biaxially textured metal substrate with reduced or no magnetism, with increased strength and better thermal expansion match to the superconductor.
The present invention provides a method for fabricating biaxially textured composite substrates with desirable compositions. This provides for applications involving epitaxial devices on such alloy substrates. The alloys can also be thermal expansion and lattice parameter matched by selecting appropriate compositions. They can then be processed according to the present invention, resulting in devices with high quality films with good epitaxy and minimal microcracking.
The terms "process", "method", and "technique" are used interchangeably herein.
For further information, refer to the following publications, the noted sections of which are incorporated herein by reference:
1. K. Sato, et al., "High-J, Silver-Sheathed Bi-Based Superconducting Wires", IEEE Transactions on Magnetics, 27 (1991) 1231.
2. K. Heine, et al., "High-Field Critical Current Densities in Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8+x /Ag Wires", Applied Physics Letters, 55 (1991) 2441.
3. R. Flukiger, et al., "High Critical Current Densities in Bi(2223)/Ag tapes", Superconductor Science & Technology 5, (1992) S61.
4. D. Dimos et al., "Orientation Dependence of Grain-Boundary Critical Currents in Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x Bicrystals", Physical Review Letters, 61 (1988) 219.
5. D. Dimos et al., "Superconducting Transport Properties of Grain Boundaries in Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 Bicrystals", Physical Review B, 41 (1990) 4038.
6. Y. lijima, et al., "Structural and Transport Properties of Biaxially Aligned YBa.sub.2 Cu.sub.3 O.sub.7-x Films on Polycrystalline Ni-Based Alloy with Ion-Beam Modified Buffer Layers", Journal of Applied Physics, 74 (1993) 1905.
7. R. P. Reade, et al. "Laser Deposition of biaxially textured Yttria-Stabilized Zirconia Buffer Layers on Polycrystalline Metallic Alloys for High Critical Current Y--Ba--Cu--O Thin Films", Applied Physics Letters, 61 (1992) 2231.
8. D. Dijkkamp et al., "Preparation of Y--Ba--Cu Oxide Superconducting Thin Films Using Pulsed Laser Evaporation from High Tc Bulk Material, " Applied Physics Letters, 51, 619 (1987).
9. S. Malhajan et al., "Effects of Target and Template Layer on the Properties of Highly Crystalline Superconducting a-Axis Films of YBa.sub.2 Cu.sub.3 O.sub.7-x, by DC-Sputtering, " Physica C, 213, 445 (1993).
10. A. Inam et al., "A-axis Oriented Epitaxial YBa.sub.2 Cu.sub.3 O.sub.7-x, --PrBa.sub.2 Cu.sub.3 O.sub.7-x Heterostructures, " Applied Physics Letters, 57, 2484 (1990).
11. A. Goyal et al., "High Critical Current Density Superconducting Tapes by Epitaxial Deposition of YBa.sub.2 Cu.sub.3 O.sub.x Thick Films on Biaxially Textured Metals", Applied Physics Letters, 69, 1795, 1996.
12. A. Goyal et al., "Conductors with Controlled Grain Boundaries: An Approach to the Next Generation, High Temperature Superconducting Wire", Journal of Materials Research, 12, 2924, 1997.
Hereinafter, each above listed publication is referred to by its number 1-12 enclosed in brackets.